More often than not, rotary regenerative air preheaters are used to transfer heat from a flue gas stream exiting a furnace, to a combustion air stream incoming therein. Conventional rotary regenerative air preheaters (hereinafter referred to as ‘preheater(s)’) includes a rotor rotatably mounted in a housing thereof. The rotor contains a heat transfer or absorbent assembly (hereinafter referred to as ‘heat transfer assembly) configured by stacking various heat transfer or absorbent elements (hereinafter referred to as ‘heat transfer elements’) for absorbing heat from the flue gas stream, and transferring this heat to the combustion air stream. The rotor includes radial partitions or diaphragms defining compartments there between for supporting the heat transfer assembly. Further, sector plates are provided that extend across the upper and lower faces of the rotor to divide the preheater into a gas sector and one or more air sectors. The hot flue gas stream is directed through the gas sector of the preheater and transfers the heat to the heat transfer assembly within the continuously rotating rotor. The heat transfer assembly is then rotated to the air sector(s) of the preheater. The combustion air stream directed over the heat transfer assembly is thereby heated. In other forms of regenerative preheaters, the heat transfer assembly is stationary and the air and gas inlet and outlet hoods are rotated.
The heat transfer assembly must meet various important requirements, such as the transfer of the required quantity of heat for a given depth of the heat transfer assembly. Additionally, there may be a requirement for low susceptibility of the heat transfer assembly to significant fouling, and furthermore easy cleaning of the heat transfer assembly when fouled, to protect the heat transfer elements from corrosion. Other requirements may include surviving of the heat transfer assembly from wear associated with soot or ashes present in the flue gas stream and blowing there through, etc.
The preheaters, typically, employ multiple layers of different types of the heat transfer elements within the rotor. The rotor includes a cold end layer positioned at the flue gas stream outlet, and can also include intermediate layers and a hot end layer positioned at the flue gas stream inlet. Typically, the hot end and intermediate layers employ highly effective heat transfer elements which are designed to provide the greatest relative energy recovery for a given depth of the heat transfer assembly. These layers of the heat transfer assembly conventionally include heat transfer elements with open flow channels that are fluidically connected to each other. While these open channel heat transfer elements provide the highest heat transfer for a given layer depth, they allow the soot blower cleaning jets to spread or diverge as they enter the heat transfer elements. Such divergence of the soot blower jets greatly reduces cleaning efficiency of the heat transfer assembly and the heat transfer elements. The most significant amounts of fouling typically occur in the cold end layer due at least in part to condensation of certain flue gas vapors. Therefore, in order to provide heat transfer elements that allow effective and efficient cleaning by soot blower jets, the cold layer heat transfer assembly is configured from closed channel elements. The closed channels typically are straight and only open at the ends of the channels. The closed channels form separate individual conduits for the passage of flows, with very limited potential for the mixing or transfer of flows with adjacent channels.
The closed channels configured by the combination of heat transfer elements in the conventional preheaters, however, may have low heat transfer effectiveness because some of the heat transfer elements may not have appropriate surface enhancement. Other closed channels configured by the combinations of heat transfer elements may have better heat transfer effectiveness, but due to sheets being tightly packed, may not allow the passing of the larger soot or ash particles. Further, if the dimensions of such heat transfer elements were altered for loosening the heat transfer assembly to allow the large soot or ashes to pass therefrom, the heat transfer elements may not be protected with a corrosion resistant coating, since the looseness allows the impinging soot blower jets to induce vigorous vibrations and collisions between elements that damage the corrosion resistant coating.
Accordingly, there exists a need for heat transfer elements and assemblies that may effectively configure closed channel elements to preclude problems of the conventional preheaters in relation to overall heat transfer effectiveness and specifically in cold end surface, soot blowing effectiveness, passing of large soot or ash particles, cleaning of the heat transfer elements and avoiding corrosions thereof.